In the current proposal, we investigate the mechanisms underlying transplantation tolerance induced by treatment with anti-CD45RB antibody. This agent is of considerable interest based on a number of attractive properties, including: 1) that a short course of anti-CD45RB results in long-lived transplantation tolerance in experimental animals, 2) that it may be unique in its ability to reverse ongoing rejection and still result in tolerance, 3) that it may synergize with costimulatory blockade, and 4) that it may exert a negative regulatory signal to T cells through up regulation of CTLA-4. Because of these unique attributes, recently there has developed a renewed enthusiasm for evaluating humanized or human anti-CD45 reagents in primate preclinical testing as a component of a regimen to induce transplant tolerance. Despite extensive study at the cellular level on the CD45's role in T cell activation, the mechanism of tolerance induced by anti CD45RB antibody in vivo is poorly understood. This fact is underscored by two novel and unexpected observations we recently made regarding its in vivo action: first is that tolerance induced by anti-CD45RB is thymus dependent and acts through generation of antigen specific thymus derived T-regs; and second is that tolerance depends on the presence of host B lymphocytes. These findings may make this agent unique among tolerance inducing antibody regimens. Moreover, the fact that anti-CD45RB induced tolerance is centrally mediated further heightens interest in its potential as a unique agent for clinical application. To integrate our preliminary findings into a cohesive model explaining the tolerogenic property of anti-CD45RB, we develop a model to explain the B cell dependent state of central tolerance induced by this agent. Studies in the proposal that dissect the immunological mechanism by which anti-CD45RB therapy leads to transplantation tolerance are facilitated by use of a TcR transgenic model of allograft rejection developed in our lab. This approach allows precise definition of the activity of graft specific T cells and antigen specific T-regs in vivo. Further understanding of the tolerogenic properties of anti-CD45RB therapy in the context of our preliminary data will not only provide insight into the action of this potentially clinically relevant agent, but also may define novel pathways of tolerance induction in vivo.